Milk production, and thus milk quality, is of economic importance to Ireland and therefore herd management factors that are associated with milk quality need to be accurately quantified. Hence, the objective of this study was to quantify the association between herd management factors and bulk tank SCS. Because of the design of the study, it should be noted that the associations reported within do not imply cause and effect, and should not be interpreted as such. Nonetheless, this study provides an insight into the proportion of a random, but representative sample of Irish farmers that undertake different management practices as well as describing farm management practices associated with bulk tank SCS. Median SCC of the farms in the present study was 282,887 cells/ml, which is similar to the geometric mean of 250,937 cells/ml accounting for an annual increase of 5,000 cell/ml reported by Berry et al. [6] for a large number of Irish dairy herds in 2004.
Rodrigues et al. [22] documented that from the bulk tank milk samples taken on Wisconsin farms there was a small prevalence of contagious pathogens, and the type of facility was not associated with types of pathogens recovered. In contrast, Barkema et al. [4] documented that there was a relationship between different pathogens and management practices. Backema et al. (1999) also reported that factors associated with bulk tank somatic cell count were related to the incidence rate of clinical mastitis caused by S. aureus. Also, Rodrigues et al. [22] reported that many pathogens were identified from those farm bulk tank samples, while not to a single pathogen was identified from the bulk tank milk samples in the current study. Pitkälä et al. [20] also identified many pathogens from milk samples of cows within confinement systems of either stanchion barns or loose housing. Barkema et al. [4] showed that the incidence rate of S. aureus clinical mastitis was related to factors associated with bulk milk SCC. S. aureus cure rates are variable with a decrease in cure as SCC, duration of infection, number of quarters infected and age of the cow increase [5].
Herd management
The lower SCS observed in herds that practice milk recording is possibly due to increased farmer knowledge on individual cows and its importance as a factor associated with SCC was substantiated by its persistence in the multiple regression model. Hutton et al. [13] also reported that farmers that were more aware of the mastitis status of the herd had lower SCC.
The beneficial association between the use of dry cow therapy and lower SCC is probably due to minimising the carry over effect of subclinical mastitis across lactations [16]. Smith et al. [24] also showed that dry cow therapy reduced the rate of streptococcal infections during the early dry period but had no effect during the prepartum period. Wenz et al. [25] showed a trend between the use of dry cow therapy and low bulk tank SCC.
Teat preparation and hygiene
In agreement with most previous studies [3, 7] clean farms, houses and milking parlours were strongly associated with lower SCS. The importance of cleanliness and hygiene was substantiated by these factors remaining in the multiple regression model. Nonetheless, no significant association was observed in the present study between cow cleanliness and SCS, which disagreed with Reneau et al. [21] who reported lower SCC in cleaner cows in The Netherlands. Schreiner and Ruegg. [23] also reported an increase in SCS and prevalence of intramammary environmental pathogens as udder hygiene score increased within a scale of one to four, four indicating dirty cows. The cleaner the roadway and holding yard; the less chance of dirt splashing on the cow's udder both before and after milking which may reduce the exposure of the teat ends to manure. Schreiner and Ruegg [23] reported that the primary sources of exposure for environmental mastitis pathogens to the cow are the presence of moisture, mud, and manure. The more sanitised the machine, the fewer bacteria transmitted to the first line of cows from the last line of cows in the previous milking. This is increasingly important in herds with high milk SCC and mastitic cows at the end of milking. A higher frequency of passageway cleaning and cubicle cleaning and also specific bedding material types were associated with bulk tank SCC. In agreement, Chassagne et al. [7] showed that cleaner dry cow sheds were observed more frequently in the lower SCC category. Also, Barkema et al. [3] reported that the cleanliness of cubicles and a greater frequency of cubicle cleaning were associated with lower bulk milk SCC. Wenz et al. [25] documented that both the bedding material and the housing facility for cows were associated with bulk tank SCC.
Teat preparation is well researched for its association with SCC and intramammary infection rate but the results differ slightly, with the current study finding no association between pre-milking teat preparation and SCS, which is in agreement with Hutton et al. [13] who reported that there was no significant difference in numbers of herds using teat preparations in the study between high and low SCC groups. However, in contrast it has also been shown that the utilisation of pre-milking teat preparation compared with no teat preparation is significantly associated with lower bulk tank SCC [12], reduced presence of bacteria [19] and reduced incidence of new intramammary infections [10, 18]. Goldberg et al. [11] concluded that insufficient hygiene prior to milking may repress the effect of improved management practices. The difference in results between the current study and previous studies may be due to cows in the present study being milked while at pasture and therefore being less dirty and under less pathogenic load, with the subsequent effect of reducing any potential benefits of teat preparation as may be observed in confined cows. Barkema et al. [3] found the use of teat disinfection to have a reducing effect on bulk milk SCC and Chassagne et al. [7] showed that teat spraying was more predominant in the low SCC group, both of which support the current study.
Milking parlour
This study showed a difference between recorder plants and direct pipelines with regard to SCS, the explanation of which requires additional information on the parlour design, as it is unknown if the vacuum was affected or the milk line height was different on these farms. The farms with automatic cluster removers had lower SCS, potentially due to consistent cluster removal at a specific milk yield and less chance of over milking. Natzke et al. [17] showed that the increase in new infections from over milking is due to an increase in the number of quarters infected in an already infected cow rather than the number of newly infected cows increasing. Hutton et al. [13] reported that cluster removers were less frequent on high SCC herds than low SCC herds. Wenz et al. [25] also documented that the use of automatic cluster removers was associated with lower bulk tank SCC. In agreement with the present study, Hutton et al. [13] also reported that milking clinically infected cows last was more common in low SCC herds.